351 running on 6 under 2000 revs - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars

472gt · 09-22-2005, 01:06

351 clevo running on six under 2000 revs until u apply gas then it picks back up 2 8, does not break down at high revs changed coil ,plugs leads have been thru the basics and not having much joy ive heard the duraspark dizzy setup is known 4 probs like this any ideas?

TruBlu351 · 09-22-2005, 01:10

Can you isolate it to the 2 bad cylinders?

ie: take the leads off at idle and see which ones don't make it worse.

Is it ROUGH causing you to say 6 cylinders or 2 are dropping out? ie: could it be carby etc?

Other things.........Dissy cap/rotor?

RPO-083 · 09-22-2005, 03:13

I've had this exact problem on cars fitted with a Carter Thermoquad. The little quad rings start to leak, and the engine basically floods at low revs. Normally only one will leak, so that gives you 4 cylinders straight away (dual plane manifold), and due to the different runner lengths between the middle two cyliders and the end two cylinders, the miss only shows up on two......

472gt · 09-22-2005, 12:22

Quote:

Originally Posted by RPO-083

I've had this exact problem on cars fitted with a Carter Thermoquad. The little quad rings start to leak, and the engine basically floods at low revs. Normally only one will leak, so that gives you 4 cylinders straight away (dual plane manifold), and due to the different runner lengths between the middle two cyliders and the end two cylinders, the miss only shows up on two......

this sounds about right like its flooding until u stamp on the gas what do u mean by quad rings do the 780 holleys? would blown power valve cause this?

472gt · 09-22-2005, 12:29

Quote:

Originally Posted by RPO-083

I've had this exact problem on cars fitted with a Carter Thermoquad. The little quad rings start to leak, and the engine basically floods at low revs. Normally only one will leak, so that gives you 4 cylinders straight away (dual plane manifold), and due to the different runner lengths between the middle two cyliders and the end two cylinders, the miss only shows up on two......

do the 780 holleys have a setup like this ?as this sounds very similar 2 what it is doing flooding until u stamp on the gas. could a blown power valve in carby cause these symptons ?

472gt · 09-22-2005, 12:33

Quote:

Originally Posted by TruBlu351

Can you isolate it to the 2 bad cylinders?

ie: take the leads off at idle and see which ones don't make it worse.

Is it ROUGH causing you to say 6 cylinders or 2 are dropping out? ie: could it be carby etc?

Other things.........Dissy cap/rotor?

it is probably running rough more so than dropping 2 cylinders unable 2 isolate the cylinders. have new cap and rotor on also

TruBlu351 · 09-22-2005, 23:44

Blown power valves generally make your car idle really RICH, and the exhaust makes your eyes water like cutting onions!

You can try running the mixture screws right in (LEAN) and look for any improvement. If your car still runs (with mixture screws all the way in), then it could be a leaky power valve, or the throttle plates are a too far open (idle screw) and the transition circuit is providing fuel.

RPO-083 · 09-23-2005, 01:34

Quote:

Originally Posted by 472gt

this sounds about right like its flooding until u stamp on the gas what do u mean by quad rings do the 780 holleys? would blown power valve cause this?

Nah, different setup between Thermoquad and Holley. The Carters have little square o-rings (quad rings) between the top and middle housings to seal the fuel passage to the main jets. When one leaks, you get overfueling to two (sometimes four depending on how bad a leak) cylinders only, which gives a miss.

The Holley basics have been posted above; progressive ports, power valve, etc.

Freshmeat · 09-23-2005, 06:37

Rebuild the carb and be done with it. It costs about $25(US) and it only took me 45 minutes to rebuild the Holley 598 that was on my Mustang. It's easy and you can get very specific instructions on how to do it. End of problem.